In the processing of threads composed of capillary glass fibers it is desirable to minimize the number of contacts between the thread and associated guide or feed members disposed along its path, especially members causing a deflection. Thus, the fragility of the glass strands generally prevents the use of roller pairs designed to advance the thread at a prescribed rate, unless the fibers are coated with rubber or the like as taught in U.S. Pat. No. 3,538,699, for example. In practice, therefore, the feed velocity of plain glass-fiber threads is determined by the rotary speed imparted to the supply bobbin by its driving mechanism as well as by the radius of the bobbin at the point where the thread is being pulled off. This radius, however, is not constant but diminishes progressively during unwinding and, usually, also varies over the length of the bobbin.
To minimize the change in the linear thread velocity occurring with a given rotary speed during the unwinding process, which is turn leads to an irregular twist of the thread or yarn being wound up, it is customary to use supply bobbins of relatively large core diameter wound with a thread layer of relatively small thickness. This procedure has several inconveniences, including the considerable moment of inertia of the bobbin which retards its acceleration to the full operating speed at the beginning and its deceleration at the end of the unwinding process as well as when rotation is to be temporarily halted in the event of a thread break. Other drawbacks are the need for a wider spacing of adjacent working stations and the frequent replacements required by the limited length of thread initially carried on such a bobbin.
In textile machinery, as well as in the somewhat remote field of wire drawing, it is known to control the speed of a take-up spool to maintain a prescribed linear feed rate. German laid-open application No. 22 18 063, published Dec. 21, 1972, describes a process and a device for varying the rotary speed of a wind-up spool by controlling its drive motor in response to an output signal from a device measuring yarn speed. German printed specification No. 25 35 457, published Feb. 10, 1977, discloses a somewhat similar arrangement according to which the winding speed is controlled by varying the speed of a drive motor in response to a signal proportional to the power consumption of a synchronous motor operating a feed roller. From German patent No. 591,837 of Jan. 27, 1934 it is further known, in an apparatus for transferring wire from a supply reel to a take-up reel, to drive the latter reel at a constant speed and to vary the rotary speed of the supply reel, under the control of an articulated linkage gauging the wire tension between the two reels, in a manner compensating for the progressive increase in the radius of the wire layer on the take-up reel and the progressive decrease in the radius of the wire layer on the supply reel.
The application of the last-mentioned arrangement to a ring spinner or twister in textile machinery would not solve the problem of requiring a large diameter for the core of the supply bobbin and a relatively thin thread layer thereon as discussed above. Nor would a controlled variation of the spindle speed, by itself, keep the linear thread velocity constant since that velocity depends inter alia on the lag of the frictionally retarded traveler on its guide ring with reference to the spindle rotation. In a yarn-plying operation, however, constancy of twist requires the maintenance of a predetermined relationship between feed rate and spindle speed.